The present invention relates to an apparatus for contacting a particulate solid material with a gas. More particularly, the invention relates to an apparatus where a fluidized powder in a bunker is introduced into a vertical column or tower and falling powder is brought into contact with a gas therein.
Gas-solid contacting operation is industrially important for the purposes of gas-solid catalytic reaction, gas-solid reaction, drying of solid particles, modification of particle surface, etc. These operations are realized in various types of apparatus, i.e. fixed bed, moving bed and fluidized bed. In a fast fluidized bed, a powder and a gas flow upward cocurrently in a column at a high gas flow rate, whereas a powder is operated in batch wise in ordinary fluidized bed. On the other hand, a powder falling type gas-solid contactor has been known as a type of cyclone, and it has been used for the purposes of gas cleaning as reported in C. J. Stairmand, Trans. Inst. Chem. Eng., vol. 29, 15-44(1951) or N. Cherrett, J. Inst. Fuel, vol. 35, 245(1962), drying and burning of powder as reported in H. Seidl, A.S.M.E. and I.M.E. Joint Conf. Combust., USA Boston (June 1955), or smelting and reduction of ore as disclosed in U.S. Pat. No. 2,973,260 to Y. Nogiwa.
Since the powder can be used in a relatively dilute concentration in the fast fluidized bed and the powder falling cyclone type apparatus, they are advantageous, as compared to the ordinary dense phase fluidized bed, in terms of easy handling of a powder having poor flowability such as cohesive powder, or gas-solid contact time control. The powder falling type apparatus is especially convenient for low upward gas velocity operation.
In a powder falling type gas-solid contactor having features as mentioned above, powder charge and gas discharge at the top of the solid contact column and powder discharge and gas supply at the bottom of the column are essential operations, thus the powder is countercurrently contacted with the gas to achieve good performances of the gas-solid contactor. It is important to control adequately the feed and discharge of the solid and gas. As for the method of the powder feeding, gravitational falling from the bottom of a hopper type bunker to a column and mechanical feeders are known for the powder feeding. It is recently reported that the fluidization of the powder in the bunker by supplying a fluidization gas significantly enhances the powder discharge rate through an orifice provided at the bottom of the bunker [J.A.H. de Jong; "Vertical Air-Controlled Particle Flow from a Bunker through Circular Orifices", Powder Technology, Vol. 3, 279-286(1970)]. In this fluidization assisted operation, a constant powder discharge rate is maintained by keeping a steady fluidized bed height in the powder bunker. To keep the steady fluidized bed height, powder can be added to the fluidized bed from a hopper above the bed through a standpipe. In this method, it is also reported that the upper surface of the fluidized bed is kept at the same level as the lower end of the standpipe.
In the practical operation of the above mentioned powder falling gas-solid contacting method, several difficulties are encountered as follows: First, it is difficult to feed a powder from the hopper to the fluidized bed in the bunker through the standpipe so as to stably keep the height of fluidized bed at the same level as the lower end of the standpipe. The fluidized bed height often tends to increase to higher than the level of the bottom end of the standpipe, thus resulting in increase of the feed quantity of powder into a vertical gas-solid contacting column. If very narrow standpipe is used to regulate the excess powder supply, the height of the fluidized bed often decreases, thus resulting in decrease of the powder feed quantity. As a result of the change of fluidized bed height, the powder flow rate in the vertical gas-solid contacting column varies from time to time, thus a stable gas-solid contacting operation is impaired. Secondly, in the vertical gas-solid contacting column, the powder tends to fall through a relatively dense stream line beneath an orifice provided at the bottom of the fluidizing bunker rather than dispersing uniformly to all cross sectional area of the column. This uneven powder flow prevents the good contact between gas and solid as compared to the uniformly dispersed flow. Finally, the powder falling from the fluidized bed into the column always accompanies the fluidizing gas cocurrently. This makes it difficult to feed a contacting gas countercurrently, thus the gas-solid countercurrent contacting efficiency is lowered.